In a wavelength division multiplexing (WDM) transmission system, various different information channels are encoded, i.e. modulated, into light at different frequencies, i.e. different wavelength channels. Typically continuous wavelength light is generated at a particular frequency, modulated with some kind of modulator, which encodes the information into the light, and then combined with other optical channels at different light frequencies using a multiplexer. The combined light is transmitted through an optical fiber and/or an optical fiber network to a receiver end of the optical fiber. At the receiver end, the signal is separated, i.e. demultiplexed, back into the individual optical channels through a de-multiplexer, whereby each optical channel can be detected by some optical detector, e.g. photo-diode, and the information can reconstructed on a per-channel basis.
While propagating through the optical fiber, light tends to loose intensity due to the losses related to the physics of how the light interacts with the optical fiber. Yet some minimal level of optical channel intensity is required at the receiver end in order to decode information encoded on the optical channel. In order to boost the optical signal while propagating in the optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The amplifiers extend the maximum possible length of the link, e.g. from a few hundred kilometers to several thousand kilometers, whereby after each fiber span, the optical signal is amplified to power levels close to the original levels at the transmitter. During the amplification process some amount of noise is introduced which prevents links from being of unlimited length.
The amplifiers at amplification nodes should similarly amplify all optical wavelength channels, which are propagated in the link; otherwise, some channels will not have sufficient intensity and signal-to-noise level at the receiver end, resulting in information being lost. A typical communication link 5, schematically illustrated in FIG. 1, includes a transmitter 10 for generating the optical wavelength channels and multiplexing the channels into a single WDM signal, and a plurality of spaced apart optical amplifiers 11, e.g. Erbium Doped Fiber Amplifiers (EDFAs) separated by fiber spans 13. The number of spans 12 and amplifiers 11 will vary from link to link. The WDM signal is demultiplexed back into wavelength channels and then separately detected at the end of the link 5 at a receiver 13.
Optical fibers in communication links introduce optical dispersion, which has undesirable effects on the performance of the link. Typically, Dispersion Compensation Modules (DCMs) are inserted at amplifier nodes of the link, between stages of EDFAs, in order to compensate the link dispersion and thus to improve the link performance. Moreover, additional optical components, such as add/drops, cross-connects and DGEs (Dynamic Gain Equalizers) may also be inserted in the middle of an amplifier, requiring multiple controlled gain stages in the amplifier to compensate for the loss due to the additional optical components.
A multi-stage EDFA 20, illustrated in FIG. 2, comprises first and second controlled gain stages 21 and 22, and first and second optical amplification sub-stages 23 and 24. In the first control stage 21a first variable optical attenuator (VOA) 31 is situated after the first optical amplification stage 31, and a second VOA 32 is embedded within the second optical amplification stage 32. Accordingly, the second VOA 32 is located between EDF (Erbium Doped Fiber) coils that produce amplification. A DCM 38 or other optical device is positioned in between the first and second controlled gain stages 21 and 22.
Portions of the light are deviated from the main optical link by taps 41, 42, 43 and 44 into photo-detectors 51, 52, 53 and 54, respectively, for measuring the light's power before and after the first and second optical amplification sub-stages 23 and 24. The information needed for gain control is passed by electrical signals 61, 62, 63 and 64 from detectors 51, 52, 53 and 54, respectively, into a master controller 75. The detectors 51, 52, 53 and 54 are calibrated in such a way that an accurate representation of the power at various parts of the amplifier gain stages 21 and 22 can be determined by the measurements performed thereby.
The amplifiers 11, i.e. the first and second controlled gain stages 23 and 24 can be Raman optical amplifiers, distributed or discrete, or a combination of EDFA and Raman amplifiers. During Raman amplification, pump light is launched into the optical fiber via the first and/or second controlled amplifier stages 23 and 24, and signal amplification occurs in the fiber spans 12. The pump light can be launched either co-propagating with the WDM signal or counter-propagating therewith. The pump light can consist of multiple wavelengths to achieve desired signal amplification characteristics. The internal portions of each amplifier 11, such as dispersion compensation module containing long portions of the fiber can also be pumped for Raman amplification.
One effect of light propagation through the communication fiber is inter-channel Raman interaction, which manifests as tilt in the transmitted spectra, i.e. the wavelength channels with shorter wavelengths have lower power than the wavelength channels with longer wavelengths, after propagation through the fiber. The spectral tilt depends on both total optical power and wavelength channel distribution. Conventional optical amplifiers 11 have tried to compensate for the Raman spectral tilt effect by introducing a gain tilt of the opposite sign.
The communication links described above are so called point-to-point links, in which all information is transmitted from one point only to another point. However, in a realistic transmission system there are multiple points that need to transmit information and multiple points that need to receive information. Different optical channels, which originated at the same transmitter 10, are required to go to different receivers situated at different locations. Instead of simple point-to-point optical communication links, more complex, network-type or web-type topology is used, in which optical channels are switched from one path to another path at multiple network nodes, which are referred as cross-connect nodes and add/drop nodes.
The process of switching the channels at multiple network nodes results in the number of channels passing through each optical amplifiers 11 to vary with time. In order to keep the channel power at the output of each amplifier 11 constant over time, regardless of the number of wavelength channels passing through, the pump power of the first and second controlled amplifier stages 23 and 24 needs to be adjusted to compensate for the changes in the wavelength channel load, which is called “transient control”. Amplifiers 11 with transient control are called either transient controlled amplifiers or gain controlled amplifier, i.e. the control is achieved by monitoring and keeping the average gain of the amplifier constant. Failing to do transient control results in the signal power significantly varying at the receiver 13 over time and over wavelength, which could result in some of the transmitted information being lost.
During a transient event, conventional transient controlled amplifiers adjust the pump power in the first and second controlled amplifier stages 23 and 24 to compensate for variations in input signal power by keeping the average amplifier gain constant; however, conventional transient controlled amplifiers do not compensate for Raman tilt variations with time when channel loading and total power is changing.
While some Raman tilt compensation techniques have been developed to compensate for different steady state loads, most require measurement of the per channel power by an optical channel monitor (OCM) or other similar device. Due to the relatively long time for OCM devices to perform measurements, accurate tilt compensation is not possible during fast transient events.
An object of the present invention is to overcome the shortcomings of the prior art by providing an optical amplifier which compensates for Raman tilt during and after a transient event.